大清河流域土地利用变化的地形梯度效应分析

地形对于土地利用具有重要制约作用。该研究选取1974-2019年的Landsat影像,从地形起伏度、坡度、地形位等级的角度,分析大清河流域土地利用类型、结构变化在地形梯度上的分布效应,对影响研究区土地利用变化的驱动力进行讨论。结果表明:1)大清河流域土地利用类型分布具有显著的地形梯度。建设用地、耕地与水体在低地形梯度地区分布优势明显,草地的优势分布区在中、高地形梯度地区,林地的优势分布区多集中在高地形梯度区域。2)大清河流域土地利用结构变化类型主要包括保持恒定与后期变化2种。其中保持恒定型以低地形梯度的耕地与高地形梯度的林地为主;后期变化型广泛分布于中高、中低地形梯度地区,建设用地挤占耕地现象愈加严重,耕地的优势分布区向更高的地形梯度区域扩张,因为"退耕还林还草"战略的实施,林地的优势分布区呈现下移趋势,耕地由单向转入演变为双向流转。3)由于人口的增长与降水减少等原因,大清河流域在太行山麓毁林垦荒约4866km~2,后由于城市扩张导致建设用地侵占耕地约3727km~2,"人地矛盾"尖锐、建设用地总体设计不合理、集约化程度不高等问题突出,未来土地政策应根据不同地形梯度因地制宜。

Analysis of terrain gradient effects of land use change in Daqing River Basin

Dynamic evolution of land use is manifested in the continuous change of land use types in time or space. Previous studies reported that the land use type changes with terrain slope and surface undulation, showing an obvious gradient. Taking the Daqing River Basin of China as the research area, this study aims to analyze the terrain gradient of land use change using Landsat images in 1974, 1988, 2001, and 2019. The map of land use type was obtained in different periods using the object-oriented and the decision tree classification combined with a small amount of manual interpretation. The changing pattern of land use types was investigated in the topographic gradient, including the topographic undulation, slope, and topographic level. The driving forces were determined for the changes in the land use structure from the views of natural conditions, social economy, and government decision-making. The results are as follows. 1) There was a significant topographic gradient distribution of land use types. Construction land, arable land, and water bodies exhibited obvious advantages in low terrain gradient areas. The dominant distribution of grassland was in the middle and high terrain gradient areas, while that of forest land was mostly concentrated in high terrain gradient areas. 2) Two types included for the changed land use: the keeping-constant type and the changing-later type. The keeping-constant type included mainly cultivated land with low terrain gradient, and the forest land with high terrain gradient, whereas, the changing-later type distributed widely in middle-high and middle-low terrain gradient areas. The condition for the construction land that occupied farmland was becoming much more serious, while the dominant distribution of cultivated land had been expanding to higher terrain gradient areas. There was a downward trend in the dominant distribution of forest land, whereas, the cultivated land evolved from one-way transfer to two-way transfer, due mainly to the strategy implementation of the Returning Farmland to Forests and Grassland. 3) Large areas of land reclamation occurred in the foothills of Taihang Mountains during the economic backwardness period, due to the dual effects of the continuous decrease in precipitation and population growth. The cultivated land was converting into construction land, forest land, and grassland with the coordinated development in the Beijing-Tianjin-Hebei region. The contradiction between cultivated land and construction land was becoming increasingly sharp with the implementation of cultivated land protection policy. 4) Rational measures were needed for local conditions in the next stage of development. The plain areas with low topographic levels were for the cultivated land and construction land. Therefore, the speed that construction land encroached on cultivated land should be slowed, while the scale and structure of urban/rural construction land should be optimized to promote land-intensive development. The primary areas of soil erosion were concentrated in the sloping fields at the middle and low terrain levels in the foothills of Taihang Mountains, where the implementation of the Three-North Forest Protection Project and the Returning Farmland to Forests and Grassland needed to be strengthened. The scale of cultivated land should be strictly restricted in those medium and high terrain level mountainous areas. Simultaneously, nature reserves should be established under adequate consideration of ecological benefits. The finding can provide a scientific basis for the adjustment and optimization of the regional land structure.